The present invention relates to an optical fiber.
A silica-based single-mode optical fiber (to be referred to as an SMF (Single-Mode Fiber) hereinafter) which is generally used in optical communication has a wavelength band for giving the minimum transmission loss within the range of 1.4 to 1.6 .mu.m. Such a wavelength band is preferably used for long-distance optical communication. However, when an optical signal having the minimum transmission loss wavelength propagates through the SMF, the waveform degrades due to chromatic dispersion, resulting in limitations on the bit rate and the transmission distance.
The chromatic dispersion in such an optical fiber is given by both material dispersion and waveguide dispersion. For example, in a conventional SMF having a core diameter of 10 .mu.m, whose relative index difference .DELTA. between the core and cladding is 0.3%, material dispersion is more dominant than waveguide dispersion. Since the chromatic dispersion of silica used as a material is reflected to result in a zero-dispersion wavelength in the 1.3-.mu.m band, the SMF used in a wavelength band of 1.5 .mu.m of large-capacity optical communication has a chromatic dispersion of about +17 ps/nm/km.
Note that "+17 ps/nm/km" means that when an optical pulse having a spectral width of 1 nm (FWHM) propagates in a 1-km long optical fiber, the pulse width broadens by about 17 ps ("Nonlinear Fiber Optics", Govind P. Agrawal, p. 63 (Dispersion-induced Pulse Broadening), Academic Press).
Conventionally, demand has arisen for a technique of reducing the limitation on transmission capacity due to chromatic dispersion to increase the bit rate and transmission distance. To meet this requirement, an optical fiber called a dispersion shifted fiber (to be referred to as a DSF (Dispersion Shifted Fiber) hereinafter) has already been developed as an optical fiber having minimum chromatic dispersion in a communication wavelength band of 1.5 .mu.m (Nobuo K. et al., "Characteristics of dispersion-shifted dual shape core single-mode fibers", J.L.T., LT-5, No. 6, p. 792 (1987)).
In this DSF, the index distribution of the core and cladding is designed such that waveguide dispersion has an opposite sign to that of material dispersion but the same absolute value. The zero-dispersion wavelength is set within the 1.5-.mu.m band. To satisfy these conditions, the relative index difference .DELTA. between the core and cladding is set to be 0.7% or more, i.e., the waveguide dispersion is made large. However, when the relative index difference .DELTA. is large, the core diameter must be small to satisfy the single-mode condition (to be described later). Consequently, the field distribution of light becomes narrow, and the effective core area (to be referred to as an Aeff hereinafter) is smaller than that of the SMF.
The single-mode condition will be described. In case of a step-index fiber, letting .lambda. be the wavelength to be used, a normalized frequency V at the wavelength to be used is given by: EQU V.DELTA.=(2.pi./.lambda.).multidot.a.multidot.n.sub.1 (2.DELTA.).sup.0.5( 1 ) EQU .DELTA.=(n.sub.1 -n.sub.2)/n.sub.1 ( 2)
where a is the core diameter, n, is the refractive index of the core, n.sub.2 is the refractive index of the cladding, and .DELTA. is the relative index difference between the core and cladding. To satisfy the single-mode condition, the value of the frequency V must be 2.405 or less.
When the relative index difference .DELTA. is increased to make the waveguide dispersion large, the core diameter a must be designed to be small instead. However, when the core diameter a is reduced to increase the relative index difference .DELTA., the light confinement effect in the core increases. The Aeff becomes smaller than that of the SMF, and additionally, the bending loss decreases.
A transmission system with a regenerative repeater spacing of 320 km and a bit rate of 10 Gb/s has already been put into practical use by applying the DSF (Dispersion Shifted Fiber) to the transmission line and an erbium-doped optical fiber amplifier (to be referred to as an EDFA hereinafter) to the repeater device.
As a technique of increasing the transmission capacity, wavelength division multiplexing (to be referred to as WDM hereinafter) has conventionally received a great deal of attention domestically and internationally. With the WDM, a plurality of signal wavelengths can be simultaneously used in one communication optical fiber. This realizes a transmission system having a larger capacity than that of the conventional single wavelength transmission.
As described above, when the DSF is used as the transmission line, the intensity of light in the optical fiber (i.e., optical power per unit area of the fiber section) becomes high because of the small Aeff. On the other hand, along with the increase in intensity of signal light, phenomena called optical nonlinear effects are likely to take place in the optical fiber in general. Especially, the effects are easily induced in the DSF having a high intensity of light.
The optical nonlinear effect which decreases the S/N ratio is a serious problem because it imposes considerable limitations on the bit rate and transmission distance of the transmission system using the DSF. Therefore, an actual transmission system using the DSF must transmit signals while suppressing the gain of the optical amplifier.
However, as the bit rate rises, the time slot per signal bit becomes short. To ensure the received power level, signal power per bit must be increased. This does not agree with suppression of optical nonlinear effects. To suppress the optical nonlinear effects, transmission power must be reduced to limit the bit rate.
When the WDM is employed to increase the transmission capacity, optical nonlinear effects called four-wave mixing (to be referred to as FWM hereinafter) are induced because of presence of a plurality of wavelengths in the optical fiber, so the bit rate and transmission distance are limited.
In the FWM, the third-order nonlinear optical process causes interference between signal wavelengths to generate new light. As the phase matching condition between wavelengths is satisfied, the FWM generation efficiency increases. For this reason, the FWM is more likely to take place when the signal wavelengths are closer to the zero-dispersion wavelength, and the interval between signal wavelengths is smaller. In the DSF whose zero-dispersion wavelength is within the signal wavelength band, the FWM is more likely to be induced than in the SMF, so the interval between signal wavelengths must be increased. However, since the amplification bandwidth of the EDFA is about several ten nm, a large wavelength interval decreases the number of signal channels to limit the transmission capacity.
The application purpose of the DSF is not limited to the transmission line.
For the further improvement of the transmission system, extensive studies on a high-speed optical switch and wavelength conversion device have also been made. The optical switch and wavelength conversion device perform switching or wavelength conversion using the optical nonlinear effects, unlike the transmission line, so how to induce the optical nonlinear effects is the important problem.
An optical switch and wavelength conversion device which are realized using the DSF in which the optical nonlinear effects readily occur because of the small Aeff have already been reported.
However, at a bit rate of 20 Gb/s or more, electrical signal processing cannot be used, and instead, the optical switch or wavelength conversion device must be used. The DSF to be used for the optical switch or wavelength conversion device must have a length of several ten km because the conversion efficiency is low. In addition, input optical power is required.